function  [x1,y1]=numcross(x,y,Pc)
if  rand < Pc
    lam1=0.4;
    x1 =lam1* x + (1-lam1) * y;
    y1 =lam1* y + (1-lam1) * x;
else
    x1 =x;
    y1 =y;
end

